If it's just the bead blasted finish you're after, then you want to take the finish to whatever you would for a "hand rubbed" or "satin" finish. The blasted finish will not take out/cover scratches. The only instance I've found where I can get away with "less" of a finish is when I apply a "stone washed" finish with a vibratory tumbler. In that case I can stop at 400 grit on the grinder, then blast the blade, etch it, then go into the tumbler (leaving the "sludge" from the etch on the blade).
For most others, I go to a hand rubbed finish of 600 or more before blasting. Much depends on the blast media a person chooses to use. Straight, heavy grit aluminum oxide, or Silicon carbide grits can "cover" 320-400 grit scratches, but it takes a "good" blast gun and some time. Glass bead media is the least expensive, but it requires a finer finish. If you miss scratches then blast with glass beads, it will normally cause the scratches to stand out.